Evaluation of the Bulk Lifetime of Silicon Wafers by Immersion in Hydrofluoric Acid and Illumination

Abstract

We report on a procedure to temporarily attain a very high level of surface passivation for silicon wafers at room temperature. When applied during a photoconductance measurement, the procedure permits an accurate assessment of the bulk lifetime, even for high-lifetime samples (τbulk > 5 ms) that are otherwise difficult to measure. It is already established that the surfaces of a silicon wafer can be well passivated by immersion in hydrofluoric acid (HF). Here, we show that the HF passivation is greatly enhanced by illuminating the wafers just prior to measurement, and that the HF passivation depends critically on the surface preparation, where the best passivation is attained after etching the wafers in tetramethylammonium hydroxide. We assess the level of passivation for a range of HF concentrations and wafer resistivities, and we demonstrate that S < 5 cm/s can be attained on 0.8–1000 Ω-cm n- and p-type silicon wafers. We demonstrate the value of the method with two examples.

Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects

“…The lower lifetime achieved by the SiN x film on the p-type sample is due to depletion region recombination caused by the positive charge contained within the SiN x film. In contrast, depletion region recombination, if present, does not appear to significantly affect the lifetime measurement of either n-or p-type silicon when using the HF passivation technique 9,17,18 . This also makes the technique desirable for analyzing bulk defects, because any injection dependence observed from the lifetime measurement can be attributed to bulk recombination and not surface.…”

“…The successful implementation of the bulk silicon lifetime measurement technique described above is based on three critical steps, (i) chemically cleaning and etching the silicon wafers, (ii) immersion in a 15% HF solution and (iii) illumination for 1 min 17,18,19 . Without these steps, the bulk lifetime cannot be measured with any certainty.…”

“…Optimization of the HF solution was examined in our previous publication 17 . It was found that when silicon wafers are immersed in 15%-30% HF, the lowest surface recombination is attained.…”

Evidence for Vacancy-Related Recombination Active Defects in as-Grown N-Type Czochralski Silicon

“…The thermal stability of grown-in defects was then investigated using minority-carrier lifetime measurements with a room-temperature surface-passivation technique [15]. In this technique, silicon wafers were prepared by etching the samples for 10 min in 25 wt% tetramethylammonium hydroxide (TMAH) at 60-70°C and, subsequently, cleaned using RCA1 at ∼70°C for 10 min.…”

“…This etch and clean procedure was performed before each passivation round in order to remove surface defects and to ensure a low surface recombination velocity (S). Following the TMAH etch and RCA 1 clean, the silicon wafers were then immersed in a plastic container filled with 150 mL of 20 wt% HF and subsequently illuminated to activate the passivation, as described by Grant et al [15]. The method allows bulk lifetimes well above 1 ms to be reliably measured.…”